A superconducting rotating machine using a superconducting winding, which causes a superconducting phenomenon, is typically an electric motor or a power generator composed of a stator, a rotor, and a housing supporting the rotor and the stator. Notably, examples of rotating machines other than a superconducting rotating machine include a normal conducting rotating machine using a normal conducting winding which does not cause a superconducting phenomenon. A superconducting rotating machine generally has a radial gap type structure in which a rotor is made superconducting (superconducting field winding) and a stator is made normal-conducting (normal conductive armature winding).
A stator in a normal conducting rotating machine is composed of an iron core made of a ferromagnetic material such as iron, and a stator winding disposed in a concave-shaped groove, which is formed on the iron core and called a slot. The iron core is formed by stacking alloy steels having small magnetic hysteresis and high saturation magnetization, such as an electromagnetic steel sheet. According to the iron core thus disposed, a magnetic flux from the rotor is converged to the iron core, whereby a magnetic field strength as well as rotation force can be increased.
On the other hand, the stator of the superconducting rotating machine uses an air-cored structure having no iron core. The reason of this is as follows. The superconducting rotating machine has high magnetic flux. Therefore, if an iron core is used, the superconducting rotating machine is easy to reach a saturated magnetic flux state, and further, an eddy current loss is easy to occur at the iron core part. For example, PTL 1 describes a structure of a so-called full superconducting rotating machine in which both a rotor and a stator are made superconducting, but an air-cored superconducting winding is disposed on an inner surface of the cylindrical stator in a circumferential direction, different from the above normal structure.
Notably, it has been known that, even if a stator of a superconducting rotating machine uses an air-cored structure having no iron core, it has a copper loss and an eddy current loss as main losses, similar to a stator of a normal conducting rotating machine.
A copper loss occurs as electric resistance heat due to a current flowing through a stator winding. In order to reduce the copper loss, a technique in which a cross section of a stator winding is increased to suppress a current density in order to reduce an electric resistance has been known. For example, PTL 2 discloses that a copper loss generating on a stator is reduced by increasing a space factor of a winding in a slot.
On the other hand, an eddy current loss occurs as electric resistance heat caused by an eddy current generated around a magnetic flux. For example, PTL 3 describes that a stranded wire formed by twisting a plurality of thin strands is disposed at an inlet part of a slot through which a large amount of leakage magnetic flux passes, and a bundle wire in which a plurality of thin strands is not twisted is disposed at a rear part of the slot in which a gap enlarges by disposing the stranded wire. Notably, the bundle wire in PTL 3 is provided not to reduce an eddy current loss but to increase a space factor of a winding in a slot.